U.S. Pat. No. 7,238,317 describes a method, a twin-wall pipe and an apparatus of the generic type. The greater the nominal widths of corrugated pipes, the more grow the elevations and thus the increase in size of the pipe socket in relation to the inside diameter of the twin-wall pipe. This is due to the fact that the standard twin-wall pipe is very often used as a spigot, meaning that a twin-wall pipe is inserted by its elevations into the socket. The transition portions between the twin-wall pipe that leads during in-line production and the pipe socket on the one hand, and the pipe socket and the lagging twin-wall pipe on the other, possess considerable radial extension. In particular the transition portion between a twin-wall pipe and socket, which remains after separation of the extruded continuous run of pipe, must possess pronounced radial extension i.e., must be directed steeply outwards in relation to the central longitudinal axis, so that, upon insertion of the spigot into the socket as far as to the transition portion, there will be no dead space, nor considerable dead space, where dirt might deposit. The greater the nominal widths and/or the higher the production rate, the greater the risk that the internal tube does not adhere by its full face to the external tube in the vicinity of the transition portion and at the beginning and end of the socket. In order to achieve a full face adherence and thus welding of the internal tube to the external tube the transition section is vented, here, in the area between the internal tube and the external tube into an adjacent elevation by the external tube, in the area of the transition section, being provided with at least one overflow channel passing through the adjacent trough and extending in the direction of the central longitudinal axis. From this solution, which is perfect in its beginning, it has become apparent that under disadvantageous product circumstances the overflow passage does not always show a sufficiently free cross-section for the desired ventilation.
It is an object of the invention to embody a method, a twin-wall pipe and an apparatus of the respective species in such a way that the overflow passage always has a sufficiently free cross-section.
According to the invention, this object is attained by the method of the generic type, in which a≧b refers to the radial outer height b of the overflow passage relative to its width a in the circumferential direction of the external tube.
This object is further attained by the twin-wall pipe of the generic type, in which a≧b refers to the radial outer height b of the overflow passage relative to its width a in the circumferential direction of the external pipe.
Furthermore, this object is achieved by the apparatus of the generic type, in which a≧b refers to the radial outer height b of the recess relative to its width a in the circumferential direction.
The core of the invention is to make the overflow passage and—as a reason for this—the recesses in the half shells in such a way that the overflow passage has a sufficiently big and free through-flow cross-section, wherein, advantageously, the walls are designed distinctively rounded and wherein it is arranged for the forming of the overflow passage to be carried out particularly intensively in the area of the recess of the half shell each by applying partial vacuum.
It can further be advantageous to extend the radial gap width adjacent to the overflow passage such that plastic melt is not squeezed into the over-flow passage.
Further features, advantages and details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing